DEP: Python 3.7 support

[snowman2]

Python 3.10 is coming in October ref. I like the idea of only supporting the 3 latest minor releases of Python. So, thinking about pyproj 3.2.x being the last series to support python 3.7.

Community references:

• NEP-29 says numpy dropped support for Python 3.7 in December 2021.
• pandas (Drop support for Python 3.7 pandas-dev/pandas#41678)
• xarray will likely drop support soon (DEP: Drop Python 3.7 Support pydata/xarray#6138)
• opendatacube is Python 3.8+ already ref
• django ref
• rioxarray (DEP: Drop Python 3.7 support corteva/rioxarray#451)
• geopandas (Drop Python 3.7 geopandas/geopandas#2391)